Revealing mistake: As Fester looks at the mirror, it shatters. After he asks a question, it falls off the wall. After it does, you can see what appears to be someone's fingers pushing from the other side to knock the mirror down. That spot on the wall is obviously fabric covering a hole.

Although it was kept a secret for years, the part of thing was performed by Ted Cassidy. He sometimes had to travel to the studio to do it, even if Lurch wasn't in the episode. A stagehand performed the part if both Lurch and Thing appeared in the same shot.